The present invention relates to a photocatalytic material and a photocatalytic article employing a TiO2 crystal system.
Hitherto, known materials exhibiting a photocatalytic action include the likes of TiO2 (titanium dioxide), CdS (cadmium sulfide), WO3 (tungsten trioxide), and ZnO (zinc oxide). These photocatalytic materials are semiconductors, absorb light to form electrons and holes, and present various chemical reactions and bactericidal actions. However, because titanium oxide is nontoxic and is superior from the standpoint of stability to water and acid, so far only titanium oxide has been put to practical commercial use as a photocatalyst.
However, because of the values of the band gap (Eg=3.2eV) of titanium oxide, the operating light of such a titanium oxide photocatalyst is limited to ultraviolet light with a wavelength xcex less than 380 nm. As a consequence, there remains an unfulfilled demand for development of materials which exhibit catalytic activity when irradiated with visible light with a wavelength of 380 nm or longer. These materials are desired, for example, for use indoors and for improving photocatalytic activity.
As described in Japanese Patent Laid-Open publication No. Hei 9-262482, by modifying materials using ion implanting of metal elements such as Cr (chrome) and V (vanadium) in anatase type titanium oxide having a high catalytic activity, the light absorbing edge of titanium oxide can be shifted to the long wavelength side to permit the operation of titanium oxide catalyst in visible light. No reports discussing the doping of Cr, V, and so on have been published since the early 1970s which succeeded in operating under visible light. Japanese Patent Laid-Open publication No. Hei 9-262482 describes that operation under visible light can be enabled through use of special techniques for doping Cr, V, and so on.
Thus, in the above conventional example, the operation of TiO2 photocatalyst under visible light is made possible by a technique of ion implanting metal elements in TiO2. However, metal ion implantation is disadvantageous because of its high cost. While there is a demand for methods for manufacturing TiO2 photocatalyst, such as by synthesis in solution or by sputtering, when these methods are employed, the resulting photocatalysts can not be operated under visible light. It is generally considered that this is because Cr of the dopant aggregates or forms oxides such as Cr2O3 in a crystallization process. Thus, in the conventional examples, there is a problem that a technique of ion implanting metal elements must be adopted in order for metal elements to be used to enable operation of TiO2 under visible light.
It is an object of the present invention to provide a photocatalyst which can operate under visible light by using novel materials and without using expensive techniques such as ion implantation.
In view of the above situation, the present inventors realized the present invention after conducting theoretical study of state density and optical physical properties using the first principle calculation as well as experimental study of photocatalysts reacting to light in a wavelength region extending from ultraviolet through visible light.
That is, the photocatalytic material according to the present invention comprises titanium compound Tixe2x80x94Oxe2x80x94X obtained by at least one of substituting an anion X at a plurality of oxygen site of titanium oxide crystals, doping an anion X between lattices of a titanium oxide crystal, and doping an anion X in grain boundaries of polycrystalline aggregate of titanium oxide crystal.
As a product of the above, a photocatalyst which exhibits photocatalytic activity after absorbing visible light can be obtained. Thus, the photocatalyst can exhibit satisfactory photocatalytic activity even under solar or fluorescent light.
Moreover, it is preferable in the Tixe2x80x94Oxe2x80x94X of the present invention that the anions be an element or molecule containing at least one of B, C, P, S, Cl, As, Se, Br, Sb, Te, and I. With these anions X, a new energy band is formed within the band gap of titanium oxide, permitting absorption of visible light.
Still further, it is preferable in the Tixe2x80x94Oxe2x80x94X of the present invention, that there be a chemical bond between titanium Ti and anions X. This results in charge-transfer between Ti and X and formation of an energy band, permitting efficient absorption of visible light.
Furthermore, it is preferable that TiO2 crystals are formed on the external surface side of Tixe2x80x94Oxe2x80x94X of the present invention. With such a configuration, internal photocatalytic materials are able to absorb visible light to produce electrons and holes, with a result that photocatalytic action is exhibited by the TiO2 crystals at the surface. The resulting photocatalyst can use visible light as operating light with maintaining functionality similar to conventional TiO2 photocatalysts. For example, this constitution is very advantageous for decreasing the contact angle of water to realize a hydrophilic property.
Moreover, it is suitable that the Tixe2x80x94Oxe2x80x94X of the present invention be mainly oriented along the C axis direction at its surface. This constitution permits efficient light absorption at the surface because of anisotropy of optical absorption characteristics of photocatalytic materials.
In addition, as the crystalline phase of Tixe2x80x94Oxe2x80x94X to realize these characteristics, any combination of single crystals, polycrystals, or amorphous Tixe2x80x94Oxe2x80x94X may be used. However, single crystals and polycrystals tend to exhibit a greater photocatalytic activity than does amorphous Tixe2x80x94Oxe2x80x94X.
Further, in the Tixe2x80x94Oxe2x80x94X photocatalyst of the present invention, any crystal form of anatase, rutile, and brookite may be employed as the basic crystal.